Here's a reply from another post asking about boom vangs which should answer your question.

Boom vangs are used on most dinghys and monohulls to control the leech of their mainsail. On Hobies, we have a mainsail traveler which spans the width of the boat and then our mainsheet, which if all used correctly can do the same thing as a boom vang. Basically, when going downwind, let the traveler out all the way to the end and the mainsheet tension will then control the twist on the leech of the main. Usually medium to loose tension works best sailing downwind.

I guess then that it is for doun wind only .would you adjust the main sheet before or after you let the travel car out? I find when I sheet in and the car is out all I do is move the travel car tward the center. did that make sense?

Last night I pulled up the sail for the first time, everything checks out ok.
I attached the ratchet block to the traveler car. Then I threaded the main sheet through the jam cleat on the cross bar. Is this correct?

Replay:One line goes from through the boom block then through the ratchet block then trough the traveler car then through the jam cleat?

I guess I am wondering if you use a second line just to control the traveler?

You use the bitter end of the Mainsheet line to control the traveler car. Run the end of the rope trough the swivel cam cleat that is on the rear cross bar, then trough the traveler (between the two rollers) and finally tie it off with a stopper knot using the eyelet that is riveted on the back of the rear cross bar. Let me know if you have any questions, All of the Hobie's are rigged the same with the traveler.